Here is a collection of known issues with Wall Worm, 3ds Max and/or the Source Game Engine.
Many DirectX shaders are crashing 3ds Max 2021, including the Wall Worm Blend Materials
There is a regression in 3ds Max 2021 that causes Max to crash when using Wall Worm's DirectX shaders for displacements.
Some of the MacroScripts in Wall Worm now have a default secondary action when executed with the SHIFT key held down.
Information on why keyboard shortcuts using the SHIFT key open the online documents for the function rather than running the function.
Fix: Update to the latest NVidia drivers.
The Max viewport can freeze and become unresponsive with Nitrous in DX 11.
Sometimes WW Pro will have a problem exporting a model with a Morpher modifier if the normals cache is not being displayed properly.
Do not use bitmap filetypes that Max doesn't know how to read.
Using unknown bitmap types on world geometry and displacements will crash the VMF Exporter. This currently happens if using VTFs in materials in 3ds Max 2013 since there is no VTF plugin yet.
Understanding Displacement Blend Texture Scale
This page explains one cause for displacement materials to be scaled incorrectly in Source as well as the solution. You should use bitmaps of equal dimensions in both materials controlling the Blend material.